PREFACE. vil 
testuceous* mollusea are concerned), which has not passed 
through my own hands, preferring to offer a less numerous 
list than some of my predecessors, rather than have any 
share in perpetuating error. The departures from this rule 
are, I think, only five in number, viz.:—(1) Rissoa abyssicola, 
a species established by Forbes on specimens dredged by 
him in Loch Fyne, and therefore, certainly authentic; and 
(2) Isocardia cor; (3) Arca lactea ; (4) Littorina neritoides ; 
(5) Scalaria communis, four species whose size and well- 
marked features seem to forbid the idea of any mistake 
having arisen. 
Two conspicuous and well known species — Pecten 
Islandicus and Saxicava (Panopea) Norvegica, not included 
in this list—occur occasionally in a more or less perfect 
condition in several parts of our firth, especially in Rothe- 
say Bay. These, it is almost certain, are washed out of 
glacial clays, and are no longer living inhabitants of our 
waters, although it is still possible that the last named 
species, extending as it does on our eastern coasts as far 
south as Yorkshire, may yet be found alive in this estuary, 
where in former ages it flourished abundantly and attained 
a great size. On the other hand, three smaller species— 
Leda pygmzea, Cyclostrema (Molleria) costulatum, and 
Trochus lineatus—here find a place, although they also have 
in all probability retired from the Clyde district, and are 
only now found in a dead state, and of very rare occurrence. 
One of them, however (Leda pygmeea), appears to enjoy a 
British distribution which overlaps the Clyde, and there 
thus remains a hope that it may still exist in some of 
* The non-testaceous order, Nudibranchiata, has not received from me 
as much attention as has been given to the testaceous mollusca, and I 
have therefore been obliged to supplement my notes by additions from 
the lists already published by Alder, Robertson, Landsborough, and others, 
adding the authority for the inclusion of each species, so that the reader 
may estimate its value. 
+ This supposition is, however, hardly a safe one, as I have seen Scalaria 
Turtonz, a conspicuous and easily known shell, exhibited in otherwise well- 
named collections as Scalaria communis; and in the public museum at 
Edinburgh errors as gross and unpardonable existed quite recently. 
